Harvesting onions

Can anyone please tell me the importance of waiting for onions to flop over at the neck before harvesting? I really need the area they are growing in. Will it actually do any harm to pull them up now as they are quite large and look ready. I should have said that these are winter onions.

For spring grown onions it helps the onions store better if you wait till the stems fold over naturally. Generally winter grown onions dont store so there is no problem picking them as you wish, but they will still but on quite a bit of bulb if you can wait a month or so?

I think the rule is that if you want to pick one and eat it now it will be fine, but when they flop over, the onion bulb has completed its growth, and more importantly the traditional brown dry skin has developed which acts as packaging, keeping the onion inside moist and fresh until you want to use it. The advice in my old gardening book is to make a hammock out of some garden netting, and let them dry out outside with air getting to all sides (so always good to harvest when there's a few dry days due) and that also they should be stored in the light, although I just put them in the spare fridge last year and they kept for ages in there. The brown skin is important for storing them.